Latest Permian time saw the dramatic end of the Paleozoic Era. Most of New Mexico was uplifted above sea level, with only the Delaware basin remaining as a land-locked sea, much like the Caspian Sea today, but with channels open periodically southward to the ocean. The rocks of this waning part of the Permian are called the Ochoan Series; they show an abrupt and striking change from the underlying Carlsbad reef limestones and associated black basin-filling limestones up into the laminated gypsum-anhydrite of the basal Ochoan rocks, the Castile formation. Normal marine conditions ended almost instantaneously. Excess of evaporation lowered the water level of the inland sea; the accumulated brine (concentrated salty sea water) killed the life on and near the Capitan reef, and thick beds of anhydrite were precipitated. The lowest beds of the Ochoan Series, the Castile Anhydrite, and the overlying Salado Salt, mostly filled the deep depression that was the Delaware basin; the upper beds, the Rustler Dolomite and Dewey Lake Redbeds, lap over the edges of the basin and in places rest irregularly upon the Capitan limestone.
These are unusual rocks. The Castile (about 1800 feet thick) is thinly banded, with thicker bands (laminae, thin layers) of light-gray gypsum-anhydrite alternating with thin laminae of dark-brown calcite ([fig. 9]). This lamination is believed due to annual changes, the brown calcite being precipitated during the summer and the gray anhydrite during the winter. On the surface, the calcium sulfate mineral is gypsum, but at depths of about 600 feet, these laminae are anhydrite. Addition of water to anhydrite has changed it to gypsum wherever ground water penetrated the laminae.
The Salado Salt, about 2000 feet thick, is almost entirely of rock salt (halite), with important interbeds of potassium-rich minerals—red sylvite, gray langbeinite, brownish bitter-tasting carnallite, and pale-red polyhalite. As all these salts are highly soluble in water, the Salado Salt nowhere “crops out” at the surface. East of Carlsbad, however, the potash-rich beds are mined underground, and supply about ninety per cent of the United States’ production—used chiefly as fertilizer.
The arid period of Salado Salt evaporation changed slightly as the dolomites and anhydrites of the Rustler Dolomite were laid down in the last drying moments (geologically speaking) of the Permian. Then as the seas retreated to the south, the fine-grained red sands and silts of the Dewey Lake Redbeds were spread as a thin blanket over the low lands basking under the hot Permian sun. This was a time of dying; whole races of vertebrate and invertebrate animals were wiped out, to be known today only from their fossil remains. As the dim unmarked episode of latest Permian time merged into the Triassic, an inkling of coming life was recorded in the rocks. The amphibians were more modern types, and they gave rise to the most striking of early land animals, the reptiles. This was the beginning of the conquest of the land by the reptiles, which culminated later in the dinosaurs, and was aided by the retreat of shallow seas from the continents, a change survived chiefly by the species adapted to living on land.
MESOZOIC ERA
The Mesozoic Era dawned in New Mexico on extensive plains, except for a northwest-trending range in the extreme north-central part of the state. During this, the early part of the Triassic Period (180 to 230 m.y. ago), sands and muds eroded from New Mexico were carried westward to northeastern Arizona where they now form the Moenkopi Formation, the brilliant reds and purples of the Painted Desert region. Uplands arose in late Triassic time in southwestern New Mexico. Along with mountains in south-central Colorado, these highlands were torn apart by water and wind, and the detritus was swept into sheets of brightly colored sand and shale. These beds are thickest (about 2000 feet) along the New Mexico—Texas line east of Roswell and in west-central New Mexico (near Grants) extending westward into northeastern Arizona. The eastern Triassic rocks are the redbeds of the Dockum Group with the lower Santa Rosa Sandstone and the upper Chinle beds. The northwestern rocks are the Chinle Formation overlain by the redbeds of the Wingate Sandstone.
The Chinle Formation is of special scenic interest as its beds contain the silicified trees so well shown at Petrified Forest National Monument. These varicolored rocks—red, purple, green, and gray—also decorate the Painted Desert area, the wide valley of Rio San Jose east of Laguna, and flank Interstate 40 (U.S. Highway 66) from the Texas line westward almost to Clines Corners. As some beds are weathered “ash” beds, the highlands were sites of volcanoes that spread their dust over much of the Southwest. In contrast to the underlying marine Paleozoic rocks, these Triassic beds were deposited on land by streams and in shallow lakes. Thus the beasts that roamed New Mexico were amphibians—such as the thick-skulled stegocephalians—and reptiles of the crocodilelike clan, the phytosaurs. The silicified trees in the Chinle are mostly primitive pines; some grew to heights of more than 100 feet and measure 7 feet in diameter.
New Mexico was featureless rolling prairie, with scattered low hills in the northwest, during most of Jurassic time (135 to 180 m.y. ago). In the late part of the period, the Sundance-Curtis sea and its shoreline lagoons spread down from the north into northwestern New Mexico. Sand dunes on its southeastern shores consolidated into the cross-bedded Entrada Sandstone; its reddish brown cliffs rim the Rio San Jose Valley near Grants and Gallup. In an extensive lagoon, or salt-water lake that covered most of northwestern and north-central New Mexico, the gypsum and limestone of the Todilto Formation were precipitated; this gypsum is the bed mined near Rosario siding (seen between Albuquerque and Santa Fe) by the Kaiser Gypsum Company, and near San Ysidro on White Mesa by the American Gypsum Company. The Todilto salty basin was overwhelmed by reddish sands and silts of the Summerville Formation, washed chiefly from the south, and by the multicolored sands of the Zuni Sandstone (exposed at El Morro), and then the stream and wind-blown sands and clays of the varicolored Morrison Formation were laid down in northern New Mexico. Petrified wood and bone fragments are abundant in the Morrison beds—along with uranium—but no fossil finds in New Mexico equal those in the Morrison Formation of Dinosaur National Park, Utah.
Much of North America, including southern New Mexico, was land during the Jurassic. In the streams and lakes were many kinds of fishes, amphibians, reptiles, snails, crustaceans, and water bugs; on dry land were hordes of reptiles, small primitive mammals, and ants; in the air were flying reptiles, the earliest known birds, moths, and butterflies. The earth was ruled by the reptiles, with the dinosaurs dominant—some being the most ponderous land animals of all earth history. Such were Stegosaurus and Brachiosaurus, the latter 85 feet long and weighing 50 tons.
The Cretaceous Period (70 to 135 m.y. ago) was one of great contrast in New Mexico. During Early Cretaceous time, most of the central and northern parts of the state were low lands torn by erosion, while thick piles of conglomerate, sandstone, and shale accumulated in depressions in the southwestern corner. Huge volcanoes near Lordsburg added their hot ashes, bombs, and flows to the sedimentary detritus, and thick fossiliferous limestones were laid down in muddy and sandy waters of the sea—the shoreline fluctuated over tens of miles in areas south of Lordsburg, Deming, and El Paso. These Early Cretaceous beds total 20,000 feet in thickness in some areas. In eastern and northeastern New Mexico, in contrast, thin sheets of quartz sand were deposited by streams on the edge of a shallow sea, and black muds in local lagoons.